With some water snakes its hard to tell, like for instance the northern water snake resembles a cotton a lot, the size, pattern, head shape, color. But the easiest way to tell is to look directly at the eyes if they have a slant at the top of the eye it is poisonous. If the eyes are rounded like in the pic above its not poisonous. From the first pic on the this topic its hard to see the eyes so im not sure. Or you can play with it and get it to strike and see if the fangs come out (which I don’t suggest). Most dangerous animals will avoid a confrontation with mankind unless provoked, or in fear of their lives, which then they will strike. I am a reptile person in general. I respect them a lot. They are one of the neatest creatures put on this earth.